The present invention relates to a method and apparatus for converting DC power to DC power of a different voltage from that of the original one.
FIG. 7 shows a first prior art voltage-drop type DC-DC converter 70. The DC-DC converter 70 converts an input voltage VI of a DC power supply 71 to an output voltage VO that is lower than the input voltage VI. When a transistor TR is ON, a voltage VI−VO is applied to a coil CL. The amount of change in current, ΔIL, when the transistor TR is turned on is expressed by ΔIL={(VI−VO)/L}Ton where L is the inductance of the coil CL and Ton is the ON duration of the transistor TR. When the transistor TR is turned off, a commutation diode D keeps the current flowing across the coil CL. When the transistor TR is turned off, the amount of current change ΔIL is expressed by ΔIL=(VO/L)Toff where Toff is the OFF duration of the transistor TR. When the current continuously flows across the coil CL, both current changes are equal to each other in a steady state. Therefore, the output voltage VO is {Ton/(Ton+Toff)}VI, which is smaller than the input voltage VI.
Other known types of DC-DC converters than the voltage-drop type DC-DC converter 70 include a booster type DC-DC converter and a booster/voltage-drop type DC-DC converter.
Recently, hybrid motor vehicles have been put to use in order to improve fuel efficiency and reduce the exhaust gas of motor vehicles. Hybrid vehicles use a running motor when they are started or when they run at a low speed, and use an engine when they run at a middle speed. The operational voltage for various kinds of units, such as a headlight, which a hybrid motor vehicle is equipped, is lower than the operational voltage for the running motor. The conventional hybrid motor vehicles therefore need two power supplies, a high-voltage power supply for the running motor and a low-voltage power supply for the various kinds of units.
FIG. 8 shows a prior art high-voltage and low-voltage generating apparatus 80. The apparatus 80 has an engine 51, an alternator 52, a high-voltage battery 53, a low-voltage battery 56 and a DC-DC converter 54. The alternator 52 has a three-phase AC generator 52a and a three-phase full-wave rectifier 52b which are driven by the engine 51. A high-voltage unit (motor) 55 is connected to the high-voltage battery 53. The alternator 52 generates high-voltage DC power to charge the high-voltage battery 53. The DC-DC converter 54 lowers the voltage of the high-voltage battery 53 to charge the low-voltage battery 56 and supplies the lowered voltage to a low-voltage unit 57.
FIG. 9 shows another prior art high-voltage and low-voltage supplying apparatus 90. The apparatus 90 has two alternators 52 connected to the engine 51. The two alternators 52 respectively charge the high-voltage battery 53 and the low-voltage battery 56.
The apparatus 80 in FIG. 8 needs the large-capacity DC-DC converter 54 and two batteries 53 and 56 and thus inevitably is large and heavy. The apparatus 90 in FIG. 9 is heavy and bulky because of the two alternators 52. The apparatuses 80 and 90 in FIGS. 8 and 9 are therefore unfit for hybrid motor vehicles.